This invention relates to a moving-image signal encoding apparatus which is used in video telephone and teleconference.
In general, the moving image signal has large spatial and temporal correlation. The conventional moving-image signal encoding apparatus calculates predicted error value for each pixel by making intraframe prediction and interframe prediction with motion compensation, and removes the correlation existing between the pixels. The moving image signal with the correlation removed has a narrow dynamic range, and takes particular values at a high frequency. If a proper variable length code (for example, huffman code) is assigned to the particular values which occur at a high frequency, the moving image signal can be coded to have a small amount of information.
FIG. 1 is a block diagram of the conventional encoding apparatus for the moving image signal. Referring to FIG. 1, when a digital moving-image signal is applied to an input terminal 101, a motion vector detector 102 detects a motion vector for each block which is a group of a plurality of pixels (for example, 8.times.8 pixels) by use of the reproduced pixel value of the previous frame read from a frame memory 103. A prediction circuit 104 receives the motion vector and the reproduced pixel value of the previous frame read from the frame memory 103, and makes interframe prediction with motion compensation, thereby calculating a predicted value. A subtracter 105 subtracts the predicted value from the digital moving-image signal supplied via the input terminal 101 to produce a prediction error value. An encoding circuit 106 encodes the prediction error value at each block and supplies this prediction error value code to an information source encoding circuit 107. The information source encoding circuit 107 receives positional information of the block within the current frame from a block position information generator 108, the motion vector of the block from the motion vector detector 102, and the prediction error value code from the encoding circuit 106, and makes optimum information source encoding according to the appearance probabilities of these values to produce a variable length code. A buffer 109 converts the variable length code into a bitstream. A channel encoding circuit 110 divides the bitstream into units of a plurality of bits each and produces them as cells on a transmission path 111. On the other hand, a decoder 112 locally decodes the prediction error value code produced from the encoding circuit 106, thus reproducing the prediction error value. In addition, an adder 113 adds this reproduced prediction error value and the predicted value from the prediction circuit 104, thus reproducing the pixel value. This pixel value is written in the frame memory 103.
Therefore, according to this conventional example, the moving image can be encoded to have a small amount of information. In addition, by converting the transmitted information into cells, it is possible to efficiently transmit the moving image at a variable rate.
In the conventional moving-image signal encoding apparatus, however, if a cell is lost on the transmission path, part of the corresponding bitstream is also lost. Thus, since the bitstream is a sequence of variable length codes, the code word included in the bitstream following the lost cell cannot be correctly decoded. In addition, information is lost in the bitstream following the lost cell until the unique code word which can always be recognized appears.